The Global Positioning System (GPS) is a satellite-based radio navigation system. In the GPS system, each GPS satellite, also called Space Vehicle (SV) broadcasts time-tagged ranging signals and navigation data. A GPS receiver tracks the satellites whose signals are within its field of view. From these visible satellites a GPS receiver extracts the navigation data and obtains range measurements from its received GPS satellite signals. The range measurements are used in a navigation solution to calculate a position fix of the GPS receiver.
The GPS navigation data contain, but are not limited to, satellite ephemeris, where ephemeris parameters can be used to accurately calculate satellite position and velocity. In addition to knowing the satellite position, the GPS receiver receives the range measurement to the satellite in order to calculate the position fix. Range measurements include, but are not limited to, two types of measurements—pseudorange (PR) and delta pseudo range (DPR).
A pseudorange is the apparent distance from the GPS receiver to the satellite. It is calculated by multiplying the speed of light by the apparent transit time, which is the time difference between a signal reception time based on a receiver clock and a signal transmission time based on a satellite clock. This range is termed pseudorange since the receiver clock is not synchronized with the satellite clock and thus the measured range is not necessarily the true range.
A second type of range measurement is the delta pseudo range (DPR), which in some receivers are reported in forms of carrier phase or Doppler frequency measurements. DPR is the measured pseudo range change rate. It is a function of a relative velocity, and a relative clock frequency drift between the satellite and the GPS receiver. The DPR measurement allows the receiver to calculate a receiver velocity and a receiver clock frequency drift, from which a new position fix can then be obtained if a previous position is known.
Pseudorange measurements associated with at least four satellites are typically used to obtain a position fix. This is because the receiver typically not only resolves its 3-D position, but also its clock bias since the receiver clock is not synchronized with the satellite clock. Four pseudorange measurements thus give four simultaneous equations enough information to calculate its 3-D position and clock bias. If only three pseudorange measurements are available, then the receiver can generally obtain a 2-D position fix by assuming the receiver's altitude is known. If less than three pseudorange measurements are available, then the receiver cannot compute the position fix.
Satellite ephemeredes are typically decoded from live signals before the navigation solution uses the pseudorange data to obtain a first position fix; however, it takes at least 18 seconds to collect a set of satellite clock and ephemeris parameters, and in some weak signal environments, correctly decoding ephemeredes is very difficult. Several solutions are available to reduce the time to first fix (TTFF). One such solution is the Assisted GPS (AGPS) technology which uses a GPS assistance server to assist the GPS receiver in obtaining the position fix. In one type of AGPS operation modes—Mobile Station assisted (MS-assisted), the GPS receiver sends the GPS measurement data wirelessly to the GPS server for the GPS server to calculate the receiver position. It is a solution, among others, to the Federal Communication Commission (FCC) Enhanced-911 (E911) requirement which mandates that the position of a cell phone to be available to emergency call dispatchers.
However, in the operation of the GPS receiver, there are many locations where the number of satellites available to the GPS receiver for measurement is less than four. Such situations arise in urban canyons, parking structures, tunnels, or other locations with significant signal blockage. In these situations the GPS server in an AGPS operation mode or the navigation solution in a stand-alone GPS receiver may not be able to obtain the position fix or may only obtain a very poor fix.